Radio frequency modules and modules for moving target detection

ABSTRACT

It is an object of the present invention to provide a radio frequency module incorporating an MMIC that has a high S/N ratio while ensuring a high output. 
     A radio frequency module according to the present invention incorporates an MMIC having a field effect transistor in which channel layers for traveling of carriers are formed by a heterostructure of two or more different kinds of materials, and height of a potential barrier of an interface between the different kinds of materials is less than 0.22 eV.

This is a continuation application of U.S. Ser. No. 09/793,113, U.S.Pat. No. 6,469,326, filed on Feb. 27, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radio frequency module. The presentinvention relates particularly to a radio frequency module incorporatinga monolithic microwave integrated circuit (hereinafter abbreviated to anMMIC) fabricated by using a field effect transistor capable of achievinga high S/N ratio. More particularly, the present invention relates to afield effect transistor that can achieve a high S/N ratio. Still moreparticularly, the present invention relates to a module for movingtarget detection.

2. Related Arts

For a radio frequency module, especially a module required to provide ahigh output, the following techniques are employed. For high output,there is known a technique in which a high electron mobility transistor,commonly referred to as a HEMT, which has a channel layer of high indiumcontent is used in an MMIC including an amplifier (hereinafterabbreviated to an amplifier MMIC). This technique is intended to improvedevice current drivability and increase output of the amplifier MMIC byusing a channel layer of high indium content, for example a layer ofindium, gallium, and arsenic having a high mobility and a highsaturation rate. The technique is described in the IEEE MICROWAVE ANDGUIDED WAVE LETTERS Vol.4 No.8, pp.277-278, for example.

In addition, for high output of an amplifier MMIC, there is known atechnique that uses a HEMT whose channel layer comprises a layer ofindium, gallium, and arsenic and a layer of indium and phosphorus. Thischannel structure is referred to as a composite channel. A conventionaltechnique that uses a HEMT having a composite channel in an amplifierMMIC is described in the IEEE Transactions on Electron Devices Vol.42No.8, p.1413, for example.

It is accordingly an object of the present invention to provide a radiofrequency module incorporating an MMIC that has a high S/N ratio whileensuring a high output. More technically, the present invention isintended to achieve high S/N ratio of a field effect transistor that isa main active device forming an amplifier, an oscillator, or a mixer ofthe MMIC. In addition, the present invention is intended to enable thefield effect transistor to have a high S/N ratio while ensuring a highoutput. Problems in achieving the above are factors that causedegradation in characteristics of a monolithic microwave integratedcircuit having an oscillator formed with the field effect transistor(hereinafter abbreviated to an oscillator MMIC), an amplifier MMIC, andthe like. Problems in achieving the above are factors that causedegradation in characteristics of a transmitter or a receiver havingsuch an oscillator MMIC or an amplifier MMIC. The oscillator MMICparticularly has a problem of degradation in noise characteristics,while the amplifier MMIC particularly has a problem of difficulty inachieving high output at its amplifying stage, especially at its finalstage. The amplifier MMIC also invites degradation in noisecharacteristics.

Accordingly, it is another object of the present invention to provide aradio frequency module that includes an oscillator or an amplifierhaving excellent S/N characteristics.

Various forms of field effect transistors used in conventional radiofrequency modules, typified by a HEMT, each have problems that make itdifficult to fully meet objects of the present invention. In addition toa HEMT with an ordinary structure, a HEMT with a composite channel hasbeen proposed. The HEMT with a composite channel serves an object of thepresent invention to achieve high current drivability and high breakdownvoltage.

A field effect transistor according to the present invention is intendedto prevent breakdown due to physical characteristics of its channelmaterials or to avoid a problem of increase in noise caused by thecomposite channel. In other words, the present invention is intended toavoid problems of the composite channel and achieve both high currentdrivability and high breakdown voltage.

SUMMARY OF THE INVENTION

Radio frequency modules according to the present invention have thefollowing configurations.

According to a typical aspect of the present invention, there isprovided a radio frequency module comprising monolithic microwaveintegrated circuits on a single substrate which include at least anoscillator, an amplifier, and a receiver, at least one of theoscillator, the amplifier, and the receiver including a field effecttransistor having a channel region with a junction of two or moredifferent kinds of materials.

According to another aspect of the present invention, there is provideda radio frequency module comprising an amplifier MMIC portion includingat least a field effect transistor, the field effect transistor having achannel region with a junction of two or more different kinds ofmaterials, and height of a potential barrier of a junction interfacebetween the different kinds of materials in the channel region beingless than 0.22 eV.

According to a further aspect of the present invention, there isprovided a radio frequency module comprising an oscillator MMIC portion;an amplifier MMIC portion for amplifying an output signal of theoscillator MMIC portion; a receiver MMIC portion for amplifying areceived signal; and a terminal for extracting an intermediate frequencysignal by mixing an output signal from the receiver MMIC portion withthe output signal from the oscillator MMIC portion; the oscillator MMICportion, the amplifier MMIC portion, the receiver MMIC portion, and theterminal being mounted on a single semiconductor substrate, and at leastone of the oscillator MMIC portion, the amplifier MMIC portion, and thereceiver MMIC portion including a field effect transistor having achannel region with a junction of two or more different kinds ofmaterials.

According to a further aspect of the present invention, there isprovided a radio frequency module comprising an amplifier MMIC portionincluding at least a field effect transistor; and an oscillator MMICportion including at least a field effect transistor, the field effecttransistors each having a channel region with a junction of two or moredifferent kinds of materials, and height of a potential barrier of ajunction interface between the different kinds of materials in thechannel region, experienced by conductor carriers, being less than 0.22eV.

According to the present invention, it is particularly important thatthe height of a potential barrier of an interface between the differentkinds of materials be less than 0.22 eV. Reasons for this will bedescribed later.

According to a further aspect of the present invention, there isprovided a module for moving target detection comprising monolithicmicrowave integrated circuits on a single substrate which include atleast an oscillator, an amplifier, and a receiver, at least one of theoscillator, the amplifier, and the receiver including a field effecttransistor having a channel region with a junction of two or moredifferent kinds of materials. According to a further aspect of thepresent invention, there is provided a module for moving targetdetection, wherein height of a potential barrier of a junction interfacebetween the different kinds of materials in the channel region is lessthan 0.22 eV.

The field effect transistor according to the present invention can beconfigured as various field effect transistors such as a HEMT, a MESFET(Metal Semiconductor Field Effect Transistor) and a MOSFET (Metal OxideSemiconductor Field Effect Transistor). The HEMT is especially usefulfor radio frequency applications in the present invention.

In general, such a field effect transistor is formed of compoundsemiconductor materials. A typical example of the compound semiconductormaterials is III-V compound semiconductor materials, and among others,InP compound semiconductor materials are often used.

The HEMT is a field effect transistor comprising a first semiconductorlayer containing an impurity; and a second semiconductor layer having asmaller band gap than that of the first semiconductor layer, the firstsemiconductor layer and the second semiconductor layer being joinedtogether to form a heterostructure, the second semiconductor layercontaining substantially no impurity, and the second semiconductor layeror an interface of the heterostructure functioning as a channel region.A gate electrode is disposed on the side of the first semiconductorlayer containing an impurity. A channel region formed by a plurality ofsemiconductor layers is referred to as a composite channel. As describedabove, this composite channel is useful in achieving high output. Theband gap of a semiconductor layer of the composite channel on a sidefarther from a gate electrode is generally selected to be larger thanthat of a semiconductor layer on the gate electrode side.

The gate electrode side is generally disposed on a side opposite from acrystal substrate with the channel region intermediate between the gateelectrode side and the crystal substrate; conversely, of course, thegate electrode side may be disposed on the crystal substrate side. Thus,the HEMT in the present specification may be formed by making variouscommon modifications thereto in accordance with the technical concept ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of assistance in explaining a monolithic microwaveintegrated circuit;

FIG. 2 is a sectional view of a fundamental device to be used in a radiofrequency module according to the present invention;

FIG. 3 is a block configuration diagram of a fundamental circuit of aradio frequency module according to the present invention;

FIG. 4 shows a comparison of frequency dependence of current noisespectrum densities of the fundamental device to be used in a radiofrequency module according to the present invention and a fundamentaldevice used in a conventional radio frequency module;

FIG. 5 is a diagram of a band structure at an interface of a compositechannel of a conventional field effect transistor;

FIG. 6 is a diagram of a band structure at an interface of a compositechannel of a field effect transistor according to the present invention;

FIG. 7 is a sectional view of another field effect transistor accordingto the present invention;

FIG. 8 is a diagram of a fundamental circuit configuration of anoscillator;

FIG. 9 is a diagram of a fundamental circuit configuration of anamplifier; and

FIG. 10 is a diagram of a fundamental circuit configuration of a mixer.

DESCRIPTION OF THE INVENTION

FIG. 1 shows a wafer 200 for forming an MMIC and a configuration exampleof the MMIC in perspective view. This example is one of a so-calledmicrostrip type MMIC. Various microwave circuit devices are formed on asurface 101 of a semiconductor substrate 100, while a ground conductor103 is formed on an underside 102 thereof. The semiconductor substrate100 is formed by using a compound semiconductor of GaAs and InP, or Si,for example. Active devices such as a field effect transistor and abipolar transistor as well as a resistance, a capacitance, aninductance, a transmission line or the like are formed on the surface ofthe semiconductor substrate. A grounding via-hole 110 is usually formedin the semiconductor substrate 100. An antenna 201 and a transceivermodule 202 are illustrated in FIG. 1 as being in array arrangement onthe wafer 200. FIG. 1 also illustrates a field effect transistor 104, anHBT (Hetero Bipolar Transistor) 105, a capacitor 106, an interdigitalcapacitor 107, a microstrip line 108, and a patch antenna 201.

The present invention relates to structure of a field effect transistorincluded in such an MMIC. It is to be noted that circuit configurationitself of an amplifier MMIC for amplifying an output signal of theoscillator MMIC, a receiver MMIC for amplifying a received signal, andthe like suffices to be the same as that of an ordinary radio frequencymodule. An example of circuit configuration of these integrated circuitswill be illustrated in the section of description of preferredembodiments.

By using a field effect transistor according to the present invention asa main component device of the oscillator, it is possible to preventdegradation in its noise characteristics. In addition, by using thefield effect transistor at an amplifying stage of an amplifier,especially at its final stage, it is possible to achieve a higheroutput. Moreover, it is possible to prevent degradation in noisecharacteristics of an amplifier MMIC. Furthermore, by using the fieldeffect transistor according to the present invention as a main componentdevice of a mixer, it is possible to prevent degradation in its noisecharacteristics. It is possible to obtain a high S/N ratio also in areceiver or the like using such an amplifier, for example.

As described above, a most important feature of the field effecttransistor according to the present invention is that the field effecttransistor has a channel region with a junction of two or more differentkinds of materials. Typical examples of the two kinds of channel layersinclude undoped InAsP and undoped InGaAs, or undoped InGaAs and undopedInGaP.

As described above, the band gap of a semiconductor layer of thecomposite channel on a side farther from a gate electrode is selected tobe larger than that of a semiconductor layer on the gate electrode side.The larger band gap is advantageous from a viewpoint of breakdownvoltage of the semiconductor layer. Therefore the field effecttransistor using this semiconductor layer enables a high output.However, a junction of the semiconductor layers whose band gaps differgreatly results in a potential barrier difference, and thus it isdesirable to form the junction while gradually reducing the differencebetween the band gaps. Such a junction is generally referred to as agraded junction.

When this graded junction is used in the present invention, practical asthe channel layers of the field effect transistor are an undoped InAsPlayer in which percentage of phosphorus in its composition changes from100% to 80% in a graded manner and an undoped InGaAs layer (indiumcomponent: 53%).

Also, according to another embodiment of the present invention, it isdesirable that a radio frequency module use, as a fundamental device, afield effect transistor or a HEMT that includes an undoped InAsP(InAs_(1-x)P_(x), X=0.75 to 0.85) layer in which percentage ofphosphorus in its composition changes from 75% to 85% and an undopedInGaAs layer as the channel layers of the field effect transistor.Eighty percent phosphorus in the composition is most desirable becausethe potential barrier difference between the semiconductor layersforming the junction is zero (ΔE=0).

In addition, according to a further embodiment of the present invention,a radio frequency module may use, as a fundamental device, a fieldeffect transistor or a HEMT that includes an undoped InAsP layer inwhich percentage of phosphorus in its composition changes from 75% to85% and an undoped InGaAs layer in which percentage of indium in itscomposition changes from 70% to 53% in a graded manner as the channellayers.

Thickness in a direction of lamination of each channel layer is set atabout 2 nm to 20 nm, preferably 5 nm to 10 nm, depending on requirementsfor device characteristics; however, basically the thickness may be setaccording to ordinary design.

A first embodiment of a field effect transistor according to the presentinvention will next be described with reference to FIG. 2. FIG. 2 is asectional view of a HEMT to be used in a radio frequency moduleaccording to the present invention. A typical configuration example ofMMICs is shown in FIG. 3. FIG. 3 is a block diagram showing planearrangement of the circuit. In FIG. 3, a voltage-controlled oscillator21, an amplifier 22, and a receiver 23 are each formed with the HEMTillustrated in the first embodiment as a fundamental device, and thevoltage-controlled oscillator 21, the amplifier 22, and the receiver 23form MMICs. The receiver 23 generally includes a first amplifier 27, asecond amplifier 28, and a mixer 29 that adds signals from theseamplifiers together. The first amplifier 27 amplifies a local oscillatorpower signal (LO) from the oscillator 21. The second amplifier 28amplifies a received signal (RF) from a receiving antenna 25.

For example, a signal of 76 GHz from the voltage-controlled oscillator21 is amplified by the amplifier 22, and then emitted from atransmitting antenna 24. A signal reflected from a target and returnedto the circuit is received by the receiving antenna 25 and thenamplified by the receiver. The amplified signal is mixed with thereference signal from the voltage-controlled oscillator 21, whereby anintermediate frequency (IF) signal is extracted from a terminal 26. Theextracted IF signal is used for a given signal processing system 30 tocalculate relative speed, distance, and angle of the target. AlthoughFIG. 3 does not show circuit configuration of the signal processingsystem 30 for calculating relative speed, distance, and angle of thetarget, an ordinary circuit suffices for such calculation. In mostcases, the signal processing system is provided on a substrate separatefrom the radio frequency module 203 according to the present invention.A silicon semiconductor device is used for an electric signal processingsystem in the radio frequency module 203 in many cases.

A specific example of the field effect transistor according to thepresent invention will be described in detail with reference to FIG. 2.

In this example, a GaAs substrate is used as a substrate 1. An ordinary0.6-μm thick strain relieving layer 2 for GaAs is formed on thesubstrate 1, and an undoped InAlAs layer 3 (thickness: 300 nm, Incomponent: 52%) is formed on the strain relieving layer 2 by an ordinarymethod.

Formed on the undoped InAlAs layer 3 is a so-called HEMT channel layerthat comprises an undoped InAsP layer 40 a (thickness: 20 nm) in whichpercentage of phosphorus in its composition changes from 100% to 80% ina graded manner and an undoped InGaAs layer 41 a (thickness: 20 nm, Incomponent: 53%). Incidentally, an InP substrate or the like may also beused as a substrate for crystal growth; in this case, it suffices toform channel layers of InAsP and InGaAs on the InP substrate with anInAlAs buffer layer intermediate between the substrate and the channellayers.

Formed sequentially on the undoped InGaAs layer 41 a are an undopedInAlAs layer 5 (thickness: 2 nm, In component: 52%), an N-type InAlAslayer 6 (doping concentration: 5×10¹⁸/cm³, thickness: 12 nm, Incomponent: 52%), an undoped InAlAs layer 7 (thickness: 10 nm, Incomponent: 20%), and an N-type InGaAs layer 8 (doping concentration:3×10¹⁹/cm³, thickness: 50 nm, In component: 53%). A device thus obtainedwill be referred to as a sample A in the following description.

Also, for comparison, a device having a single channel layer isfabricated. Specifically, in this example, only the channel layer isformed by an undoped InGaAs layer 4 b (thickness: 40 nm, In component:53%), and the other layers are the same as those of the sample A. Thisexample will be referred to as a sample B in the following.

Next, a so-called HEMT is fabricated on both the samples A and B by acommon process described below.

A silicon oxide film (SiO) 10 (thickness: 400 nm) is formed by a CVD(Chemical Vapor Deposition) method. Then, part of the silicon oxide film(SiO) 10 on the semiconductor substrate thus prepared is removed by anordinary photolithography process to form regions of a source electrode11 and a drain electrode 12. A hole is thereafter created in the siliconoxide film (SiO) 10 by ordinary dry etching or wet etching. Then, gold(Au, thickness: 200 nm)/titanium (Ti, thickness: 50 nm) is deposited,and a source electrode 11 and a drain electrode 12 are formed by alift-off method. Next, an opening pattern is formed between the sourceelectrode 11 and the drain electrode 12 by using an electron beamlithography apparatus.

Thereafter, a hole is created in the silicon oxide film (SiO) by dryetching, and the silicon oxide film (SiO) is deposited by the CVDmethod, so that the opening mentioned above becomes 0.15 μm. Then, anN-type InGaAs layer 9 is subjected to a wet etching process with acitric acid-based etchant, and molybdenum (Mo, thickness: 20 nm) andaluminum (Al, thickness: 500 nm) are sequentially deposited. A patternis formed by an ordinary photolithography process so as to overlay the0.15-μm opening and then milled to thereby form a gate electrode 13.Length of the gate is 0.15 μm.

It is understood from a comparison between the samples A and Billustrated in the following that the field effect transistor accordingto the present invention can achieve breakdown voltage characteristicsbetter than those of a conventional HEMT, while ensuring the same levelof current drivability possessed by a conventional HEMT. In addition,the field effect transistor according to the present invention canachieve noise characteristics that eliminate a noise produced by aconventional composite channel.

Saturation current density, which indicates current drivability of aHEMT, is about 800 mA/mm for the sample A and about 800 mA/mm for thesample B. Thus, according to the present invention, a currentdrivability value equal to that of a conventional HEMT can be obtained.On the other hand, breakdown voltage is more than 3 V for the sample Aand 3 V for the sample B. Thus, breakdown voltage of the sample Aaccording to the present invention is improved as expected.

FIG. 4 shows a result of measurement of drain current noise frequencyspectra of the samples A and B. The sample A, which is a structureaccording to the present invention, exhibits −115 dbA²/Hz at 1 kHz,while the sample B, which is a conventional structure, exhibits −115dbA²/Hz at 1 kHz. The sample B of the conventional structure has beenconsidered to excel in noise characteristics. This indicates that thesample A, a structure of the present invention, is free from noisedegradation specific to a structure using a composite channel, and thuspossesses excellent noise characteristics.

Table 1 illustrates a comparison of noise degradations specific tostructure using a composite channel. The table shows height of a barrierwithin the channel that impedes traveling of carriers, noisedegradation, and whether characteristics obtained are satisfactory for apractical radio frequency device.

TABLE 1 Barrier height in the Noise Characteristics (⊚ compositedegradation excellent, ◯ good, X channel (eV) (dB) unsatisfactory) 0.02Substantially ⊚ no degradation 0.06 0.3 ⊚ 0.1 0.45 ⊚ 0.21 1.1 ◯ 0.22 1.6X 0.4 2.2 X 0.6 3.2 X

It is shown that in the prior art composite channel comprising a layerof indium, gallium, and arsenic and a layer of indium and phosphorus hasa great barrier height of 0.22 eV, thus resulting in a noise degradationof more than 1.5 dB. The above sample A does not have a barrier thatimpedes traveling of carriers within its channel, and accordingly thesample A exhibits noise characteristics similar to those of a HEMT witha channel structure comprising a single layer of indium, gallium, andarsenic. It is understood from the results of Table 1 that when barrierheight in the composite channel is less than 0.22 eV, practicallysatisfactory and good noise characteristics can be obtained.

It is to be noted that in the present embodiment, layer thicknessresulting from epitaxial growth is fixed; however, there is no specialreason for the thickness set in the present embodiment. This is becauseit is important in the present invention to make semiconductor layersforming a channel layer of different materials and to prevent noisedegradation caused by a barrier present in an interface between thelayers and impeding the traveling of carriers.

It is also to be noted that the present embodiment uses a HEMT having achannel layer comprising an undoped InAsP layer in which percentage ofphosphorus in its composition changes from 100% to 80% in a gradedmanner and an undoped InGaAs layer (indium component: 53%); however, thepresent invention is not limited to these materials. For example,instead of the undoped InAsP layer in which percentage of phosphorus inits composition changes from 100% to 80% in a graded manner, an undopedInAsP layer in which percentage of phosphorus in its composition is 80%may be used. Alternatively, an undoped InGaAs layer in which percentageof indium in its composition changes from 70% to 53% in a graded mannermay be used. This is because according to the present invention, as longas semiconductor layers forming a channel layer are made of differentmaterials and a barrier present in an interface between the layers andimpeding the traveling of carriers is less than 0.22 eV, a value of theprior art, it is possible to suppress noise degradation and to therebyprovide a high-output amplifier MMIC.

Although it is omitted in the above description, many MMIC devices aregenerally fabricated on a single wafer, as illustrated in FIG. 1.Therefore, in mass-producing the devices, the devices are subjected toordinary photolithography and etching processes in the form of a wafer,and then separated from one another to form individual modules.

Features of the field effect transistor according to the presentinvention will next be described by comparison with the prior art.

A problem with the prior arts arises from output of an amplifier MMICbeing a function of device current drivability and breakdown voltage.For example, a decrease in breakdown voltage leads to a decrease inoutput of the amplifier MMIC, while an improvement in currentdrivability leads to an improvement in output of the amplifier MMIC.

A HEMT used in the prior art amplifier MMIC has a practical problem inbreakdown voltage of its channel material. Specifically, a combinationof indium, gallium, and arsenic with a high indium content, which can beexpected to allow high-speed traveling of carrier electrons, is used asthe channel material. Hence, as described above, the channel materialimproves current drivability, and therefore a higher output can beexpected. However, when the above HEMT is used as a radio frequencydevice for forming an MMIC, a combination of indium, gallium, andarsenic with a high indium content is used as its channel layer. Hence,the band gap of the channel layer is small, and a breakdown due toavalanche effect tends to occur at a portion beside the gate on thedrain side, where an electric field is concentrated. This results in adecrease in breakdown voltage of the device. Thus, according to theprior art mentioned above, output is actually not as high as expected.

On the other hand, a device with a composite channel has a problem inthat noise is produced by the presence of a barrier of band structure atan interface between a plurality of semiconductor layers that form thecomposite channel. A typical example of the composite channel is formedby a layer of indium, gallium, and arsenic and a layer of indium andphosphorus. In general, there is a barrier of about 0.22 eV or more atan interface between the layer of indium, gallium, and arsenic and thelayer of indium and phosphorus. This barrier impedes traveling ofcarriers, thereby resulting in an increase in noise produced by theHEMT. As a result, because of this noise, no improvement in the S/Nratio of the radio frequency module can be expected.

Characteristics of the field effect transistor according to the presentinvention will next be described with reference to diagrams of bandstructure at a heterojunction.

FIG. 5 shows a band structure at a heterojunction according to the priorart. FIG. 5 shows a band structure in a section taken in a direction oflamination of a semiconductor laminate, in which a lower end of aconduction band (Ec) and an upper end of a valence band (Ev) are shown.A composite channel in this case comprises InGaAs 31 and InP 32. Thechannel region is controlled by a gate electrode 34. A heterojunctioninterface 33 in the lower end portion of the conduction band has a bandgap difference of 0.22 eV. Thus, carriers traveling in the channel areimpeded in traveling or scattered, which consequently appears as a noisein an electric signal.

Similarly to FIG. 5, FIG. 6 shows a band structure at a heterojunctionaccording to the present invention. FIG. 6 shows a band structure in asection taken in a direction of lamination of the semiconductor laminateshown in FIG. 1. A composite channel in this case comprises InGaAs 35and InAsP 36, for example. In FIG. 6, reference numeral 38 denotes agate electrode. A heterojunction interface 37 in a lower end portion ofa conduction band has only a band gap difference of less than 0.22 eV.Thus, according to the present invention, carriers traveling in thechannel are not impeded in traveling nor scattered, or the impediment orthe scattering is negligible if any, so that noise does not appear in anelectric signal or remains at a level that does not present a practicalproblem. Incidentally, the band gap difference in FIG. 6 is shownslightly exaggerated for a better understanding.

The band gap difference is less than 0.22 eV as described above, andtherefore is better than that of the prior art. However, a band gapdifference of 0.1 eV or less is desirable, and a band gap difference of0.05 eV or less is particularly desirable.

It is to be noted that the above embodiment uses a high electronmobility transistor (HEMT), which is one type of field effecttransistor; however, the present invention is not limited to a highelectron mobility transistor. It is to be understood that the sameeffects can be obtained by applying the present invention to all kindsof field effect transistors such as a MESFET (Metal Semiconductor FieldEffect Transistor) and a MOSFET (Metal Oxide Semiconductor Field EffectTransistor).

FIG. 7 is a sectional view of a MESFET device to be illustrated asanother example of a field effect transistor.

An n-type active layer region, that is, a channel region (71 and 72) isformed on a semiconductor substrate 70, for example a GaAs substrate.The channel region in this case also forms a composite channel. Thefirst channel layer 72 is made of GaAs, and the second channel layer ismade of InGaP. A laminate of platinum and gold to form a Schottkycontact is formed as a gate electrode 74 on the compound semiconductorlayer 72. As in normal practice, laminates of gold, germanium, andnickel or the likes are used as a source electrode 73 and a drainelectrode 74. Also in the case of a composite channel of such a MESFET,a barrier at a junction interface is made less than a given value asdescribed earlier regarding the HEMT, a typical example of a fieldeffect transistor, whereby an excellent S/N ratio can be obtained.Furthermore, it is possible to achieve a higher output while ensuringexcellent noise characteristics.

Next, a radio-frequency module according to the present invention thatcomprises an MMIC fabricated with the HEMT of the present embodimentused as a fundamental device will be applied to an automotive radarmodule as an example.

A schematic plan view of an automotive radar module in this example isthe same as FIG. 3.

FIGS. 8 and 9 show fundamental circuit configurations of an oscillatorand an amplifier used in a receiver and a transmitter, respectively, foruse in the MMIC configuration. The oscillator is formed as follows. Acapacitor 43 and an inductance 44 are connected to a gate of a fieldeffect transistor 42 according to the present invention. A firstimpedance 41 is connected to a first impurity region of the field effecttransistor 42, and a second impedance 45 and a third impedance 46 areconnected to a second impurity region of the field effect transistor 42.An oscillating signal is outputted from a terminal 47 to the exterior ofthe oscillator. On the other hand, the amplifier is formed such thatfield effect transistors 52 and 54 according to the present inventionare connected in series with each other by using impedances 51, 53, and55. A signal from an input terminal 50 is amplified, and then outputtedfrom an output terminal 56 to the exterior of the amplifier.

FIG. 10 shows an example of a mixer. This example is a single-ended FETmixer. The mixer is used as the mixer 29 in FIG. 3, for example.

A received signal (RF) and a local oscillator power (LO) are inputtedfrom their respective input terminals 61 and 62, and added to each otherby an adder 63. In order to extract only an intermediate frequency (IF)component, or a frequency component representing a difference betweenthe RF and the LO, high-frequency components are removed by a low-passfilter circuit on the output side. Thus, a signal goes through an inputmatching circuit 64, a transistor 65, an output matching circuit 66, andthe low-pass filter circuit 67, whereby an IF component is extractedfrom an output terminal 68.

Of course, oscillators and amplifiers of types other than thoseillustrated in this example are conceivable. However, an object of thepresent invention can be achieved by adopting the field effecttransistor according to the present invention as a field effecttransistor used in this example. It is needless to say that it is mostdesirable to adopt field effect transistors according to the presentinvention as the field effect transistors used in this electric circuitconfiguration.

A signal of 76 GHz from the voltage-controlled oscillator 21 isamplified by the amplifier 22, and then emitted from the transmittingantenna 24. A signal reflected from a target and returned to the circuitis received by the receiving antenna 25 and then amplified by thereceiver. The signal is then mixed with the reference signal from thevoltage-controlled oscillator 21, whereby an intermediate frequency (IF)signal is extracted from the terminal 26. The extracted IF signal isused to calculate relative speed, distance, and angle of the target.Although the figure does not show a circuit configuration forcalculating relative speed, distance, and angle of the target, anordinary circuit suffices for such calculation.

It is particularly important in the present invention that the fieldeffect transistors of the oscillator, the receiver, and the transmitterincorporated in the radio frequency module be provided in accordancewith the present invention, as described above.

An automotive radar using the automotive radar module of the presentembodiment can achieve a higher output without noise degradation, andthus its SIN ratio is improved by 3 dB as compared with a conventionalradar. As a result, the detection distance and the detection angle ofthe radar are improved by 25% and 50%, respectively, as compared with aconventional radar.

The radio frequency module according to the present invention is usefulin detecting a reflected radio wave of a radio wave oscillated by themodule. A typical example of a technique for detecting a reflected radiowave of a radio wave generated by self-oscillation is a technique formoving target detection by radars for vehicles, automobiles, and thelike.

In detecting such a reflected radio wave, for example in moving targetdetection, a higher radio wave output of the transmitter and a lowernoise are particularly desired as compared with ordinary communications.The present invention makes it possible to meet both requirements of ahigher output and a lower noise.

In addition, the present invention provides not only the aboveadvantages on the transmitter side but also advantages on the receiverside. When radio systems and radar systems for moving target detectionand vehicle collision prevention receive a weak signal, performance ofthe systems can be determined by a noise occurring in a mixer within thereceiver.

According to the present invention, it is possible to reduce noiseoccurring within the receiver by using a field effect transistoraccording to the present invention in the receiver. Therefore, it ispossible to achieve better noise and S/N ratio characteristics thanthose of a conventional receiver even when a received signal is weak.The present invention is especially useful in detecting a reflectedradio wave of a radio wave generated by self-oscillation as describedabove. Thus, by applying the present invention on both the transmitterside and the receiver side, it is possible to achieve both thecharacteristics of a high output and a low noise to be ensured on thetransmitter side and the characteristic of a low noise on the receiverside. Thus, the system as a whole ensures a high output and has a highS/N ratio.

While embodiments of the present invention have been described, it is tobe understood that the present invention is not limited to theembodiments described above, and various designs may be made withoutdeparting from the spirit of the present invention. For example, thepresent invention may be applied not only to the 76-GHz millimeter waveautomotive radar of the above embodiment but also to other microwaveradio communications and millimeter wave radio communications. When thepresent invention is applied to microwave radio communications ormillimeter wave radio communications, it is possible to provide a radiosystem with a long communication distance or a large number of channelsand the like.

Since the radio frequency module according to the present invention usesa HEMT that prevents noise degradation and achieves high breakdownvoltage and high current drivability, the radio frequency module canachieve a high output and a high S/N ratio with no noise degradation.When the radio frequency module is applied to a millimeter waveautomotive radar, it is possible to provide a highly reliable automotiveradar system having a long detection distance and a great detectionangle.

According to the present invention, it is possible to provide a radiofrequency module and a module for moving target detection incorporatingan MMIC having a high S/N ratio.

Moreover, according to the present invention, it is possible to providea radio frequency module and a module for moving target detectionincorporating an MMIC having a high S/N ratio while ensuring a highoutput.

In order to facilitate understanding of the drawings, main referencenumerals are shown in the following.

1 . . . GaAs substrate, 2 . . . strain relieving layer, 3 . . . undopedInAlAs layer, 40 a . . . undoped InAsP layer (P content graduallychanges from 100% to 80%), 40 b . . . undoped InAsP layer, 41 . . .undoped InGaAs layer, 5 . . . undoped InAlAs layer, 6 . . . N-typeInAlAs layer, 7 . . . undoped InAlAs layer, 8 . . . N-type InGaAs layer,10 . . . SiO film, 11 . . . source electrode, 12 . . . drain electrode,13 . . . gate electrode, 21 . . . voltage-controlled oscillator, 22 . .. amplifier, 23 . . . receiver, 24 . . . transmitting antenna, 25 . . .receiving antenna, and 26 . . . terminal for intermediate frequency (IF)signal.

What is claimed is:
 1. A monolithic microwave integrated circuit device comprising a field effect transistor having a channel region with a junction of two or more different kinds of materials and height of a potential barrier of a junction interface between the different kinds of materials in said channel region being less than 0.22 eV.
 2. The monolithic microwave integrated circuit device according to claim 1, wherein said monolithic microwave integrated circuit device includes at least an oscillator, an amplifier, and a receiver, at least one of said oscillator, said amplifier, and said receiver including said field effect transistor.
 3. The monolithic microwave integrated circuit device according to claim 2, wherein said monolithic microwave integrated circuit device further includes a terminal for extracting an intermediate frequency signal by mixing an output signal from said receiver with an output signal from said oscillator.
 4. The monolithic microwave integrated circuit device according to claim 1, wherein said field effect transistor comprises: a first semiconductor layer containing an impurity; and a second semiconductor layer having a smaller band gap than that of the first semiconductor layer, wherein said first semiconductor layer and said second semiconductor layer are joined together to form a heterostructure, said second semiconductor layer containing substantially no impurity, and at least said second semiconductor layer forming said channel region.
 5. The monolithic microwave integrated circuit device according to claim 3, wherein the junction of said different kinds of materials is formed by a heterostructure comprising a first semiconductor layer containing an impurity and a second semiconductor layer having a smaller band gap than that of the first semiconductor layer, said second semiconductor layer containing substantially no impurity.
 6. A monolithic microwave integrated circuit device comprising an oscillator, an amplifier, and a receiver, at least one of said oscillator, said amplifier, and said receiver including a field effect transistor having a channel region with a junction of two or more different kinds of materials, wherein said field effect transistor is a fundamental device and includes, as said channel region, an undoped InAsP layer having a percentage of phosphorus that gradually changes from 100% to 80%, and an undoped InGaAs layer.
 7. The monolithic microwave integrated circuit device according to claim 1, wherein said field effect transistor is a fundamental device and includes, as said channel region, an undoped InAsP layer having a percentage of phosphorus that gradually changes from 100% to 80%, and an undoped InGaAs layer.
 8. The monolithic microwave integrated circuit device according to claim 2, wherein said field effect transistor is a fundamental device and includes, as said channel region, an undoped InAsP layer having a percentage of phosphorus that gradually changes from 100% to 80%, and an undoped InGaAs layer.
 9. The monolithic microwave integrated circuit device according to claim 3, wherein said field effect transistor is a fundamental device and includes, as said channel region, an undoped InAsP layer having a percentage of phosphorus that gradually changes from 100% to 80%, and an undoped InGaAs layer.
 10. The monolithic microwave integrated circuit device according to claim 4, wherein said field effect transistor is a fundamental device and includes, as said channel region, an undoped InAsP layer having a percentage of phosphorus that gradually changes from 100% to 80%, and an undoped InGaAs layer.
 11. The monolithic microwave integrated circuit device according to claim 5, wherein said field effect transistor is a fundamental device and includes, as said channel region, an undoped InAsP layer having a percentage of phosphorus that gradually changes from 100% to 80%, and an undoped InGaAs layer.
 12. A monolithic microwave integrated circuit device comprising an oscillator, an amplifier, and a receiver, at least one of said oscillator, said amplifier, and said receiver including a field effect transistor having a channel region with a junction of two or more different kinds of materials, wherein height of a potential barrier of a junction interface between the different kinds of materials in said channel region is less than 0.22 eV. 